1. Field of the Invention
The present invention relates to the field of semiconductor devices. More specifically, the invention relates to current sensing circuits.
2. Related Art
Sensing circuits have been in wide use for detecting the current drawn through an electrical path or circuit loop in a variety of applications. For example, often it is desirable to ascertain the charge capacity of a battery for an electronic device in order to determine and/or display how long the device can be operated by a user. A known technique for ascertaining the charge capacity of battery involves measuring the discharge current drawn from the battery during operation and correlating the magnitude of the discharge current to the charge capacity level of the battery, as is known in the art.
In general, sensing circuits determine the discharge current by measuring the voltage across a resistor (also referred to as a “sense resistor”), where the sense resistor is either connected in series with the ground path in “low-side” sensing or connected in series with the positive terminal of the battery in “high-side” sensing. In the present application, the voltage measured across the sense resistor is also referred to as the “sense voltage.” Since the sense voltage is a function of the current, e.g., discharge current, through the sense resistor, the charge capacity level of the battery can be determined from the magnitude of the sense voltage.
As electronic devices incorporate batteries capable of being recharged, it has also become desirable to sense the charge current in addition to the discharge current in order to accurately monitor and charge the battery. Consequently, bi-directional current sensing circuits have been implemented for detecting discharge current and charge current, which flows in the opposite direction of the discharge current. Known bi-directional current sensing circuits, however, are associated with a number of disadvantages. For example, a common bi-directional current sensing approach employs current mirror circuitry. Due to a number of variations, such as variations in process and temperature, for example, the elements of the current mirror are very difficult to match. As a consequence, erroneous results can be generated by such bi-directional current sensing circuits.
Furthermore, the output generated by conventional bi-directional current sensing circuits employing current mirror circuitry has significantly reduced dynamic range since a reference voltage between ground and the supply voltage (“VCC”) is used to differentiate between charge current and discharge current. In a typical arrangement, the reference voltage is set to approximately half of VCC such that output voltage of the sensing circuit which is less than the reference voltage corresponds to discharge current, while output voltage which is greater than the reference voltage corresponds to charge current. For example, where ground is zero (0) volts (“V”) and VCC is 5V, output voltage between 0V and 2.5V corresponds to discharge current while output voltage between 2.5V and 5V corresponds to charge current. According to this example, the resolution of the output voltage is reduced by factor of two, significantly diminishing the dynamic range of the sensing circuit output.
Other known bi-directional current sensing circuits have employed complex circuit components, such as CMOS-based amplifiers, to improve dynamic range. However, such complex circuits result in significantly increased components and silicon area consumption, thereby increasing expense and reducing yield, which are undesirable.